(1) Field of the Invention:
The present invention relates to a process for the production of a high-purity lactulose syrup on an industrial scale.
(2) Description of the Prior Art:
Lactulose is used as a bifidus growth factor in foods such as infant formulas and also as a senile constipation improver in drugs.
Regarding the production of lactulose, there are several known processes basis of the process in which lactulose is obtained by adding an alkalizing agent such as caustic soda, calcium hydroxide or caustic potassium to an aqueous solution of lactose and then heating the resultant mixture to effect isomerization of the lactose. Byproducts such as galactose are however formed in a large quantity if the lactose-based yield of lactulose is increased in these conventional processes. These conventional processes are therefore accompanied by the drawback that they require an additional step for the removal of such byproducts after the isomerization reaction. Although bromolysis and electrolysis are known methods for the elimination of these byproducts, they are both difficult to economically produce lactulose on an industrial scale.
It has therefore been attempted to minimize the formation of byproducts in the reaction. For example, Japanese Patent Publication No. 2984/1977 proposes a process for the production of lactulose in which in order to minimize the formation of monosaccharides--such as galactose--as byproducts, a solution of lactose is added with caustic soda in an amount of 0.27-0.53% based on the lactose, the resultant mixture is heated to a temperature of at least 70.degree. C. to form lactulose, and removal of unreacted lactose by crystallization is repeated three times. In this process, the alkali isomerization reaction is however conducted under mild conditions to suppress the formation of monosaccharides--such as galactose--as byproducts, leading to a drawback that the yield of lactulose is as low as about 20% based on the weight of the initial lactose. It is another drawback that a great deal of time and labor is required for the removal of the unreacted lactose.
Namely, when the isomerization reaction of lactose into lactulose is conducted under mild conditions, the formation of monosaccharides such as galactose can be suppressed but the degree of isomerization is low. When the isomerization reaction is conducted under certain strong conditions, the degree of isomerization is increased but more byproducts such as monosaccharides are formed. Moreover, a subsequent complex purification step is indispensable for the production of high-purity lactulose whichever reaction conditions are employed. The above isomerization process was therefore not suited for practice on an industrial scale.